This project addresses the role of host cell factor-1 (HCF-1) in regulating the herpes simplex virus (HSV) life cycle. HSV is an important human pathogen responsible for a spectrum of diseases including painful lesions, corneal scarring and life-threatening encephalitis. Productive or lytic infection in epithelial cells is initiated by the viral transcription factor VP16, which is released from the virion into the cytosol. There VP16 binds HCF- 1, translocates to the nucleus and together activates the HSV immediate-early (IE) genes. HSV also colonizes sensory neurons where it can establish latency, re-emerging as periodicepisodes of localized lytic replication. In spite of extensive analyses, the molecular roadblocks preventing lytic replication in latent neurons remain elusive. Insufficient IE gene expression is likely to be an important contributing factor. HCF-1 is found in the nucleus of most cells, acting as an essential transcriptional cofactor for cell proliferation and cytokinesis. In neurons, however, HCF-1 is cytoplasmic but moves to the nucleus in response to DMA- damaging agents, toxins or mechanical stress. The same stimuli induce latent HSV to re-enter lytic replication, implying a causal relationship between HCF-1 localization and HSV lytic replication. This proposal investigates mechanisms that control localization and function of HCF-1 in different regions of the nucleus. We focus on HPIP and Brd7, two cellular proteins that bind to conserved domains of HCF-1. HPIP is a shuttle factor that exports HCF-1 from the nucleus to the cytosol. Our studies show that HPIP is found in both cytosol and mitochondrial and in Aim 1we will the signals that allow HPIP to partition between these cytoplasmic compartments and more precisely define the localization within mitochondria. Brd7 is a chromatin-binding bromodomain protein and ectopic expression causes major changes in nucleolar structure, leading to stabilization of the stress-sensor p53 and incorporation of HCF-1 in novel nucleolar structures. Similar alterations occur when cells sustain DMAdouble-strand breaks and we hypothesize that HCF-1 and Brd7 are key players in the response to genomic insults. Aim 2 investigates the molecular mechanisms by which Brd7 and HCF-1 initiate nucleolar breakdown and Aim 3 addresses the physiological consequences with regard to cell cycle, rRNA transcription and HSV lytic replication.